1. Field of the Invention
The present invention relates to an auto-focusing device and method of a camera of an interchangeable objective lens type and, more particularly, to an auto-focusing system wherein the reflected light from an object to be photographed passes through the objective lens of the interchangeable lens and forms an image on or near a predetermined image forming plane, which image is detected for detecting the degree of out of focus, thereby controlling the lens shift by the detected degree of out of focus. The present invention also relates to an interchangeable lens itself for use in the above described type of camera.
2. Description of the Prior Art
There have been conventionally proposed various types of the above described auto-focusing method, but most of them use visible light. Therefore, when the intensity of the visible light is very weak, such as when the scenery is dark, it is very difficult to detect the focusing condition of the target object, resulting in an error control of the objective lens. Also, for detecting the image, a charge accumulating device, such as CCD, can be employed, but would result in a disadvantage that the charging period becomes long as the visible light becomes weak, resulting in long time of focus detection.
It is known in the art that the photoelectric conversion device, such as CCD or photodiode, for use in focus detection is generally more sensitive to infrared rays than the visible rays. Also, it is known in the art that the infrared rays reflect at a high rate even on a black object and also on an organic object.
From this view point, it is possible to detect, if the infrared ray are used for focus detection, the focusing condition of an object even if the object is located in a dark place, and consequently improving the focus detection ability.
But, on the other hand, if the infrared rays are used for focus detection, it takes a disadvantage that there will be a difference in focusing distance between the the infrared rays and the visible rays, due to color aberration, and such a difference varies as the type of an interchangeable lens mounted on a camera body changes. Therefore, unless a suitable correction is effected, a picture taken by an auto-focusing system using the infrared rays always results more or less in out of focus.
To remove the above disadvantage, an improved auto-focusing system is proposed in Japanese Patent Laid-open Publication (Tokkaisho) No. 57-154224, wherein a signal pin having a length corresponding to the focusing distance difference is provided to each interchangeable lens, and a focus detecting device having a means to correct the focusing distance difference by the signal pin are provided. According to the proposed device as described above, the signal pin extending from the lens must have a required strength to avoid undesirable break or bent, and also the signal pin should be positioned with a high accuracy to enable the precise correction. Also, the focus detecting device provided in the camera body will have a complicated structure, resulting in high manufacturing cost, and because of the complicated structure, it may not be able to correct the difference to the required degree due to the inaccuracy of size and positioning of each constructing part.
Furthermore, according to the auto-focusing device disclosed in Tokkaisho No. 57-154224, it is very difficult to detect the focusing when a target object is under a weak visible light and yet containing almost no infrared rays, such as under fluorescent lamp, not only when detecting the focus condition by way of vible light, but also by way of infrared rays. Moreover, if the target object has no or hardly any contrast, such as in the case of plain wall, it is very difficult to detect the focus condition even when using the infrared rays.
Another improved auto-focusing system is proposed in Japanese Patent Laid-open Publication (Tokkaisho) No. 57-150808, which is so designed that the focus condition can be detected either by the visible rays or infrared rays. The auto-focusing system according to Tokkaisho No. 57-150808 comprises two photoelectric conversion means, one for visible rays and the other for infrared rays, and a beam splitter for dividing the rays from the object in terms of amount into rays to be directed to the visible light photoelectric conversion means and to the infrared light photoelectric conversion means. An infrared cut filter is disposed between the visible light photoelectric conversion means and beam splitter for cutting the infrared rays to be directed to the visible light photoelectric conversion means. Each photoelectric conversion means is connected, through a discrimination circuit, to a selection circuit and further to a focus condition detecting circuit.
According to the auto-focusing system of Tokkaisho No. 57-150808, it is stated that when no infrared light pass filter is used to carry out photographing using visible rays, almost the same amount of light impinges on both photoelectric conversion means and, therefore, the outputs from both photoelectric conversion means have approximately the same level. Thus, the discrmination circuit discriminates that the ratio of outputs from both photoelectric conversion means is approximately equal to one, thereby actuating the selection circuit to select output from the visible light photoelectric conversion means.
It further states that when the infrared light pass filter is used to carry out photographing using infrared rays, the amount of light impinging on the infrared light photoelectric conversion means is greater than that on the visible light photoelectric conversion means. Thus, the discrimination circuit discriminates that the ratio of outputs from both photoelectric conversion means is not equal to one, thereby actuating the selection circuit to select output from the infrared light photoelectric conversion means.
The focus condition detecting circuit operates in response to the output signal selected by the selection circuit.
From a practical point of view, however, there are a variety of objects differing from one another in reflection properties. Thus, it is not always true that the ratio between outputs from the visible light photoelectric conversion means and from the infrared light photoelectric conversion means is constant with use of no infrared light pass filter. Thus, it is practically very difficult to set a level or borderline for causing selection of an output from visible light photoelectric conversion means whenever photographs are taken using the visible light. Therefore, even when the output from the visible light photoelectric conversion means is selected, there may be a case wherein the focus detection is carried out using the infrared rays, resulting in out of focus, because there is a difference in an image forming distance of the objective lens between the visible rays and infrared rays. Moreover, when taking a photograph under the A-light source (tungsten type lamp) or under the natural light, there may be a case in which the infrared rays are stronger than the visible rays. In such a case, the photographing is carried out using the visible rays, but the focus detection is carried out using infrared rays, resulting in an error in focus adjustment.
Also, according to the prior art, it is well know to emit an auxiliary light to the object, when the object is too dark to carry out the focus detection. In this case, if the object is people, and when the auxiliary light is visible light, such an auxiliary light makes people unpleasant. In order to overcome this, an improved focus detecting device is disclosed in Japanese Patent Laid-open Publication (Tokkaisho) 55-111929, which device emits, instead of visible lights, infrared rays as the auxiliary light when the target object is very dark and has less contrast. However, Japanese Patent Laid-open Publication (Tokkaisho) 55-111929 does not take any consideration to the difference in focusing distance between the the infrared rays and the visible rays, due to color aberration of the objective lens.